The typical computer controlled plotter utilizes a carrier mechanism which selectively picks up and manipulates a pen or other conventional plotter instrument. The carrier mechanism moves the instrument transversely across a plotter table upon which the paper or other material to be marked is moved longitudinally. In this manner, X and Y control in the transverse and longitudinal dimensions of the drawing plane is achieved. The carrier mechanism operatively moves the marking point of the pen on and off of the paper or other material to control the extent to which marking occurs. The working point is raised and lowered from the surface by a slight pivoting movement of the carrier mechanism. This slight pivoting movement causes the marking tip of the pen to follow a slightly curved path which is not orthogonal to the plane of the paper or other material upon which the marking occurs.
To drill round holes in the circuit board or other material, it is necessary to axially move the drill bit orthogonal into and out of the printed circuit board. Round holes, as opposed to oval-like holes, are generally preferred in most situations. The slight pivoting movement of the carrier mechanism would cause the drill bit to move in an arc and cause the hole to become slightly oval, if the drill bit were moved up and down in an arc-like manner by the pivoting movement of the carrier mechanism. The pivoting movement of the carrier mechanism would also cause the drill bit to experience increased resistance and wear from lateral side loading as it moves up and down, and might result in drill bit breakage.
The usual procedure for drilling holes of different diameters is to interchange different sized drill bits in the drill motor. This conventional approach is unsatisfactory in making the computer controlled plotter serve as an automatically controlled device capable of drilling holes of different diameters because of the difficulty involved and the size of the automatic equipment involved in changing small drill bits in a very small drill motor.
Another constraint inherent in using a plotter as an automatic hole drilling device is that the drill motor must have a shape and configuration essentially the same as a conventional plotter instrument, such as a pen, in order for the carrier mechanism to retain and manipulate it, and in order for the drill motor to be transferred to and from the carrier mechanism automatically. Another limitation is that the operative drilling force must be coupled to the drill motor from the carrier mechanism in order that the carrier mechanism be unobstructed for automatically transferring the drill motor to and from it. Thus, the means for coupling the operative force to the drill motor must not restrict the normal movement and manipulation capability of the carrier mechanism.
These factors, among others, are important limitations, constraints and background information for the present invention. In applications other than the production of printed circuit boards, but where a computer controlled plotter is used as a device for automatically drilling holes in material carried by the plotter, these considerations are just as relevant.